The purpose of the project named Solar Impulse is to fly an aeroplane powered solely by solar energy captured by a multitude of photoelectric cells distributed over the entire surface of the aircraft. To achieve this challenge, the apparatus must be very light and have a large wing span, typically on the order of 61 meters, i.e. a wing span comparable to an Airbus A340 which weighs several hundred tonnes.
It goes without saying that there are enormous differences of control between these two types of aircraft. Without going into detail, it should be noted that the approach and landing phases are fundamentally different for piloting these two types of aircraft. A large heavy aeroplane, which is heavily motorised, has considerably fewer constraints during approach and the final landing phase. In fact, any crosswind, even a relatively strong crosswind, does not cause any great problems of stability, and the angle of approach can easily be corrected since the pilots have the ability to step on the gas and negotiate a slight turn. This being so, even if the technique and power of a large airliner offer a relatively high safety margin, the final approach and landing nonetheless remain complex flight phases requiring the full attention of the pilots, who have to control a large number of parameters within a short period of time.
The Solar Impulse pilot, while having to control the same parameters as in a conventional aircraft, will not have a power reserve enabling him to easily correct the path, particularly in the last hectometers preceding touchdown on the runway. Because of the extremely reduced weight and large wing span of this aircraft, it will be very sensitive to side winds and will only have a very limited possibility for manoeuvre during the final approach. It was therefore necessary to find a way of giving the pilot information relating to the angle of approach and the inclination of the wings of his aircraft.
As regards the angle of approach, it can be said that when an aeroplane wishes to land, it is normally directed towards the threshold of the runway in a straight line. However, it is of course sensitive to crosswinds which may cause it to deviate from its path. This is how, in the event of crosswind, the pilot, if he looks straight ahead, will notice that the runway is slightly to the left, or to the right relative to the nose of the aircraft. The power of the engines of a conventional aircraft will enable the pilot to correct the angle between the rectilinear path and the runway threshold.
For a Solar Impulse type aircraft, the pilot has no power reserve and he must therefore prepare the approach either by adhering as closely as possible to the straight line, or by anticipating the drift of the aircraft due to the crosswind as early as possible. To do so, the pilot, who is already extremely occupied with other approach manoeuvres, must easily be able to see the correction that has to be made to his path in order to approach the runway threshold properly. Moreover, he must also be able to know whether the wing is perfectly horizontal, because, as it is very large, it is liable to touch the runway on landing if it is inclining too far on one side or the other. It should be realised that with a wing span of 61 meters, it is practically impossible for the pilot to visualise an inclination of the wings of one or two meters relative to the perfect horizontal.